Leaching chambers made from thermoplastic materials are well known in the art and most have a number of design characteristics in common for both functional and manufacturing reasons. Some of these chambers have slotted, inwardly sloped, planar sidewalls which run up to a curved arch top and an arch-shape cross section with wide peak and valley corrugations running up over the arch. Slotted sidewall perforations may be included to provide open area for infiltration of wastewater through the sidewall into the soil surrounding the chamber. However, slots or other perforations weaken the sidewall. This weakened sidewall is undesirable because in use leaching chambers must resist the loads from both overlying soil, and from vehicles and other things traveling along the soil surface, as well as lateral load of soil on the sidewall.
One approach used to strengthen these chambers involves substantially thickening the sidewall in the vicinity of the slots as well as providing other structures for strength. Another approach included adding ribbing on the interior and exterior portions of the chamber. For example, in a commercial injection molded thermoplastic leaching chamber of the type sold by Infiltrator Systems, Inc., (hereinafter “ISI”), of Old Saybrook, Conn., ribbing on the interior and exterior of the chamber was used to strengthen the chamber. U.S. Pat. Nos. 4,759,661 and 5,511,903, both to Nichols et al., show chambers having features like those sold in commerce by Infiltrator Systems. These kind of chambers, a partial vertical cross section of which is shown in FIG. 5, generally have a nominally trapezoidal cross section of the prior art (i.e. planar side walls with a curved top). These chambers also have corrugations (comprised of peaks and valleys) running along the curve of the arch shape cross section and ribs running lengthwise and crosswise in rectangular fashion at various locations and are referred to hereinafter as ‘the ISI standard chamber design’. See U.S. Pat. No. 5,716,163 to Nichols et al. for embodiments of ribs in the chambers having ribs.
Compared to the ISI standard chambers, Quick4® chambers, sold in recent years by Infiltrator Systems, have an improved design, namely continuous curve cross sections and truncated semi-ellipse cross sections as exemplified by chambers shown in U.S. Pat. No. 7,118,306 to Kruger et al. and U.S. Pat. No. 7,189,027 (patent application Ser. No. 10/677,938) to Brochu et al. In such chambers, the corrugations are closer together. Compared to the older trapezoidal cross section chamber, the new design enables thinner walls and eliminate the need for ribs. This is because the continuous curve design distributed the load well and avoided the stress concentrations and associated design limitations of some of the older chambers. The absence of ribs provides for better nesting to the chambers. It should be appreciated that all of the foregoing patents/patent applications are owned in common with this application and the disclosures thereof are hereby incorporated by reference.
Continuous arch cross section chambers have been construct with designs which are modified in that, e.g., to make a wider chamber with more storage capacity, a nearly flat (horizontal) portion is designed into the top of the chamber. In these newer chambers, although the rib-free benefits of the continuous curve design are largely maintained, there is some diminution in the torsional stiffness, which stiffness is of some consequence in some applications and in manual handling of chambers. It is also desirable to provide the ISI standard design chamber with similarly improved properties.